Conventionally, as dryness/wetness responsive sensors, humidity sensors are known which detect humidity based on a change in the electric resistance value (impedance) or electrostatic capacitance of a sensor element (dryness/wetness responsive part). In a humidity sensor of an electric resistance type, as a dryness/wetness responsive material of a sensor element, a polymer, ceramics, or the like is generally used, and since the material is low-cost and the structure is simple, a low cost can be achieved through mass production. However, if the humidity sensor of the electric resistance type gets wet with water, the sensor element will break down, and thus, the humidity sensor of the electric resistance type cannot be used under a condition in which dew condensation may occur. For this reason, the measurement humidity range is restricted to the range of 10 to 90% RH, and it is difficult to use the humidity sensor of the electric resistance type in a low humidity environment of 10% RH or less and in a high humidity environment of over 90% RH. In addition, the humidity sensor of the electric resistance type has a large aging variation, and, since it also has high temperature dependency in many cases, a temperature correction is required. Furthermore, the humidity sensor of the electric resistance type also has problems of large variation in precision (about ±5 to 15% RH) and a long response time (30 seconds to several minutes or even more).
In a humidity sensor of an electrostatic capacitance type, a polymer membrane is generally used as the dryness/wetness responsive material of the sensor element. Accordingly, the humidity sensor of the electrostatic capacitance type has a higher response speed (normally, about several seconds to ten seconds) and higher precision/reproducibility/reliability than the electric resistance type. Though its typical measurement humidity range is 0 to 100% RH, there are occasions when the sensor element is broken down under a dew condensation condition. In addition, the humidity sensor of the electrostatic capacitance type also has a problem of higher production cost than that of the humidity sensor of the electric resistance type.
A humidity sensor of both of the electric resistance type and the electrostatic capacitance type requires an external drive power supply for driving the sensor. In addition, a conventional humidity sensor cannot detect the size of water droplets attached to the surface of the sensor element due to its sensor structure or its detection principle.
Recently, a dryness/wetness responsive sensor based on galvanic action has been developed and used as a corrosion environment sensor that is used mainly for monitoring the corrosive environment of a construction.
In a bridge and other various constructions, since its steel members are often exposed to outside, the degree of corrosion of used steel members has great influence on the durability performance. The progress of corrosion of a steel member greatly varies according to not only the properties of the steel member itself but also use environments including the amount of a corrosive material and electrolytes contained in the atmosphere and rainwater, the amount of attached rainwater, and its wet time. Thus, in order to evaluate remaining lives of constructions of this type and appropriately maintain them by inspection, repair or the like, it is preferable to continuously evaluate the corrosion status for each of the constructions or, if necessary, for the respective portions of even one construction the corrosion environments of which are considered to be different from each other.
However, since it is difficult and takes a cost to perform an inspection of the degree of corrosion of a steel member itself configuring a structure on site, technique has been developed actually in which corrosion environment sensors are attached to respective places for evaluating their corrosion environments and the degree of corrosion of a steel member are estimated and predicted based on the evaluation results For example, as illustrated in FIG. 1, the degradation of the steel member of a bridge or the like was predicted by attaching a sensor of this type to the steel member and monitoring the corrosion environment at that place.
A representative example of the corrosion environment sensor is an atmospheric corrosion monitoring (ACM) sensor that detects a galvanic current flowing between metals of different types due to a contact therebetween via water. Refer to Non Patent Literatures 1 to 4 for its structure, method of evaluating measured data, and the like. However, since the size of the conventional galvanic sensor becomes larger for compensating for its low sensitivity, it has problems of inconvenience of the handling and high price.